Antennas capable of beam steering, or pattern agility, have a variety of applications. For example, in high-speed wireless communication networks, agile antennas may assist with interference mitigation. Agile antennas also may be employed in point-to-point communication systems, weather monitoring, target tracking radar systems, adaptive beam formers, diversity receivers, direction of arrival (DoA) finders, and a variety of other applications.
Some steerable antenna systems, such as some phased array antennas, make use of phase shifters to control beam direction. Phase shifters may contribute significantly to the cost of an antenna system and may restrict performance. Other antenna systems may make use of beam forming networks, but this may also be relatively costly to implement.
One type of antenna system is an electronically steerable parasitic antenna radiator (ESPAR), sometimes also referred to as an electrically steerable passive array radiator. In ESPAR antennas, a driven antenna element (sometimes also referred to as a feed element or active element) interacts using parasitic coupling with nearby passive antenna elements. In such a parasitic coupling arrangement, the nearby passive antenna elements absorb radiated waves from the driven antenna element and re-radiate them with a different phase and amplitude. The waves radiated and re-radiated from the antenna elements interfere, thus strengthening the antenna system's radiation in some directions and weakening or cancelling the antenna system's radiation in other directions.
In ESPAR, the terminating impedance of each passive antenna element may be adjusted to control a beam direction of the antenna system. Depending on the terminating impedance of each passive antenna element, some passive antenna elements may act as reflectors, generally reflecting waves radiated by the driven antenna element, and some passive antenna elements may act as directors, generally strengthening waves radiated by the driven antenna element in a particular direction.